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adjacent-layer interaction
On a single computer, one layer provides a service to a higher layer. The software or hardware that implements the higher layer requests that the next lower layer perform the needed function.
decapsulation
On a computer that receives data over a network, the process in which the device interprets the lower-layer headers and, when finished with each header, removes the header, revealing the next-higher-layer PDU.
encapsulation
The placement of data from a high-layer protocol behind the header (and in some cases, between the header and trailer) of the next-lower-layer protocol. For example, an IP packet could be encapsulated in an Ethernet header and trailer before being sent over an Ethernet.
frame
A term referring to a data-link header and trailer, plus the data encapsulated between the header and trailer.
networking model
A generic term referring to any set of protocols and standards collected into a comprehensive grouping that, when followed by the devices in a network, allows all the devices to communicate. Examples include TCP/IP and OSI.
packet
A logical grouping of information that includes the network layer header and encapsulated data, but specifically does not include any headers and trailers below the network layer.
protocol data unit (PDU)
A generic term referring to the header defined by some layer of a networking model, and the data encapsulated by the header (and possibly trailer) of that layer, but specifically not including any lower-layer headers and trailers.
same-layer interaction
Two different computers use a protocol to communicate with the same layer on another computer. The protocol defined by each layer uses a header that is tramitted between the computers, to communicate what each computer wants to do.
segment
In TCP, a term used to describe a TCP header and its encapsulated data (also called an L4PDU). Also in TCP, the process of accepting a large chunk of data from the application layer and breaking it into smaller pieces that fit into TCP segments. In Ethernet, a segment is either a single Ethernet cable or a single collision domain (no matter how many cables are used).
OSI model layers
7 - application
6 - presentation
5 - session
4 - transport
3 - network
2 - data link
1 - physical
OSI - physical layer
Layer 1 - Defines the electrical, optical, cabling, connectors, and procedural details required for transmitting bits, represented as some form of energy passing over a physical medium.
OSI - data link layer
Layer 2 - Formats data into frames appropriate for transmission onto some physical medium. Defines rules for when the medium can be used. Defines means by which to recognize transmission errors.
OSI - network layer
Layer 3 - Logical addressing, routing, and path determination.
OSI - transport layer
Layer 4 - Provides a variety of services between two host computers, including connection establishment and termination, flow control, error recovery, and segmentation of large data blocks into smaller parts for transmission.
OSI - session layer
Layer 5 - Establishes and maintains end-to-end bidirectional flows between endpoints. Includes managing transaction flows.
OSI - presentation layer
Layer 6 - Defines the format and organization of data. Includes encryption.
OSI - application layer
Layer 7 - Interfaces between network and application software. Also includes authentication services.
TCP/IP - application layer
OSI layers - application, presentation, session
TCP/IP - transport layer
OSI layers - transport
TCP/IP - internet layer
OSI layers - network
TCP/IP - network access layer
OSI layers - data link, physical
Protocols for OSI layers 5-7 (TCP/IP layer 4)
HTTP, SMTP, POP3, VoIP
Protocols for OSI layers 3-4 (TCP/IP layer 2-3)
IPX (transport layer - SPX)
Protocols for OSI layers 1-2 (TCP/IP layer 1)
Mac Protocols
OSI Layering Benefits
less complex, standard interfaces, easier to learn, easier to develop, multivendor interoperability, modular engineering
OSI benefit - less complex
Compared to not using a model, network models break the concepts into smaller parts.
OSI benefit - standard interfaces
The standard interface definitions between each layer allow for multiple vendors to create products that compete to be used for a given function, along with all the benefits of open competition.
OSI benefit - easier to learn
Humans can more easily discuss and learn about the many details of a protocol specification.
OSI benefit - easier to develop
Reduced complexity allows easier program changes and faster product development.
OSI benefit - multivendor interoperability
Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.
OSI benefit - modular engineering
One vendor can write software that implements higher layers--for example, a web browser--and another vendor can write software that implements the lower layers--for example, Microsoft's built-in TCP/IP software in its operating systems.
Depiction of the data-link services provided to IP for the purpose of delivering IP packets from host to host
Book, page 29
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